Fraction of total dust mass f_d that is located where the dust density ρ_d is greater than a threshold density ρ_thres as a function of time after the dust particle initialisation t – t_d,init. The fraction is calculated as the rolling average over 100 yr. In addition to the simulations shown in Figs. 4 and 7, an analogous simulation of the streaming instability with doubled resolution is depicted. We choose threshold densities equal to once (solid lines) and twice the Roche density (dashed lines). In both the SIwhileVSI and the SlafterVSI simulation, Roche-unstable overdensities temporarily comprise as much as 1% of the dust mass, and overdensities that exceed twice the Roche density a slightly smaller fraction. Nevertheless, the fraction fluctuates by an order of magnitude or more over time. In contrast, in the simulations of the streaming instability with the doubled and the fiducial resolution only 0.1% and 0.01%, respectively, of the dust mass is part of overdensities greater than the Roche density, and a fraction that is less by at least an order of magnitude is contained in overdensities of twice the Roche density or more.